4Splay Trees

Splay trees are an efficient data structure for mutable dictionaries
with totally ordered keys. They were described in the paper
“Self-Adjusting Binary Search Trees” by Daniel Sleator and Robert
Tarjan in Journal of the ACM 32(3) pp652-686.

Makes a new empty splay-tree that permits only exact integers as keys
(in addition to any constraints imposed by key-contract). The
resulting splay tree answers true to adjustable-splay-tree?
and supports efficient key adjustment.